Getting numbers around a number

Question

So, I'm trying to do something similar to a paginator (list of page numbers) where the current number is in the middle or as close as can be

Every way I solve it is hard and weird, just wondering if there is a nice mathy way to do it :)

given:

• a: current page number
• x: first page number
• y: last page number
• n: number required

I want to generate a list of numbers where a is as close to the center as can be, while staying within x and y

so f(5, 1, 10, 5) would return [3, 4, 5, 6, 7] but f(1, 1, 10, 5) would return [1, 2, 3, 4, 5] and f(9, 1, 10, 5) would return [6, 7, 8, 9, 10]

Can anyone think of a nice way of getting that kind of thing?

Implemented in a probably complicated way in ruby, can it be done simpler?

def numbers_around(current:, total:, required: 5)
  required_before = (required - 1) / 2
  required_after = (required - 1) / 2

  before_x = current - required_before
  after_x = current + required_after

  if before_x < 1
    after_x += before_x.abs + 1
    before_x = 1
  end

  if after_x > total
    before_x -= (after_x - total)
    after_x = total
  end

  (before_x..after_x)
end

Answer 1

Here's something kind of mathy that returns the first number in the list (JavaScript code):

function f(a,x,y,n){
  var m = n >> 1;
  return x * (n > y - x) || a - m
                          + Math.max(0,m - a + x)
                          - Math.max(0,m - y + a);
}

Output:

console.log(f(5,1,10,5)); // 3
console.log(f(1,1,10,5)); // 1
console.log(f(9,1,10,5)); // 6
console.log(f(2,1,10,5)); // 1
console.log(f(11,1,10,5)); // 6
console.log(f(7,3,12,10)); // 3